Well logging apparatus with sonic energy means for removing mudcake from the borehole wall



Dec. 31, 1963 c. B. VOGEL 3,116,449

WELL LOGGING APPARATUS WITH SONIC ENERGY MEANS FOR REMOVING MUDCAKE FROM THE BOREHOLE WALL Filed Jan. 30, 1961 3 Sheets-Sheet 1 FIG. I

INVENTORI CHARLES B. VOGEL HIS ATTORNEY Dec. 31, 1963 c. B. VOG EL 3,116,449 WELL LOGGING APPARATUS WITH SONIC ENERGY MEANS FOR REMOVING MUDCAKE FROM THE BOREHOLE WALL Filed Jan. 50, 1961 5 Sheets-Sheet 2 EMF-9 8| 5 I: .C V II POWER SUPPLY 82 INVENTOR:

CHARLES B. VOGEL WZM ms ATTORNEY FIG. 2

Dec. 31, 1963 c B. VOGEL 3 116,449

. WELL LOGGING APPARATUS WITH SONIC ENERGY MEANS FOR REMOVING MUDCAKE FROM THE BOREHOLE WALL Filed Jan. 30, 1961 3 Sheets-Sheet 3 H3 H4 H5 GENERATOR AMMETER VOLTMETER FIG. 3

INVENTORI' CHARLES B. VOGEL QZZIWZM HIS ATTORNEY I attain Patented Dec. 31, 1963 3,116,449 WELL LGGGHNG APPARATUS WITH SONEC EN- ERGY MEANS FOR REMOVING MUDfZAKE FRQM THE BGREHQLE WALL Charles B. Vogel, Houston, Tex., assignor to Shell Gil Company, New York, N.Y., a corporation of Delaware Filed Jan. 3%), B61, er. No. 85,559 6 Claims. (Cl. 324-1) This invention pertains to well logging and more particularly to a method and apparatus for removing the mud cake from a borehole and then logging a characteristic of the formation surrounding the borehole which has been obscured by the mud cake.

In a copending application entitled Method and Apparatus for Investigating Earth Formations, Serial No. 636,565, filed January 28, 1957, now Patent No. 2,974,273, issued March 7, 1961, there is shown and described a method and apparatus for using sonic energy to remove the mud cake from a borehole wall and then logging characteristics of the formation surrounding the borehole that had been suppressed by the mud cake. The present invention is directed to a particular method and apparatus for practicing the above invention to remove the mud cake and then determine the permeability of the formation surrounding the borehole. Obviously, it would be impossible to accurately determine the permeability of the formation as long as it is covered with a mud cake.

Accordingly, it is the principal object of this invention to provide a novel apparatus for effectively removing the mud cake from the wall of a borehole and then logging the formation exposed by removal of the mud cake.

A further object of this invention is to provide a unique apparatus for removing the mud cake which includes a means for circulating a fluid under controlled conditions into the formation to replace the formation fluid and thus determine the permeability of the formation.

A further object of this invention is to provide a novel apparatus using sonic energy for removing the mud cake from a borehole wall and then pumping a fluid into the formation to displace the formaiton fluid. A fluid having a known resistance is used as the replacement fluid to assist in measuring the formation resistivity and determining the formation permeability.

In a preferred method of practicing the present invention, a rock contacting fluid is brought into contact with a portion of the borehole wall which has been irradiated with sonic energy. This is done by replacing the borehole fluid or drilling mud adjacent to the irradiated portion of the wall with a rock contacting fluid before, during, or after the irradiation.

This replacement allows the rock formation, that is exposed by the irradiation and dispersal of the mud cake, to be contacted with a fluid having a precisely known composition and properties. The borehole fluids generally contain a variety of slowly settling solid particles and often have a variable liquid-phase composition due to localized intrusions of rock formation fluids and/ or chemical reactions among the constituents of the borehole fluids. When the exposed formations are contacted with a rock contacting fluid of known composition, any measurements of properties related to the permeability and/ or the composition of the formation can be accurately evaluated as to the extent by which they are influenced by the formation properties. The present invention accomplishes this replacement by encasing both a portion of the borehole wall and a sonic generator within a cup that has its rim pressed against the borehole wall. The cup is provided with ports in fluid communication with the interior of the cup; and a means for pumping a rock contacting fluid into the interior of the cup.

When portions of the formations surrounding a well are exposed by a dispersal of the mud cake, and the exposed formations are contacted with a fluid of selected composition, many tests are possible. For example, aqueous liquids exhibit a behavior different from that of liquid hydrocarbons in their penetration into certain formations. By the present process, the formations at a given depth can be tested in respect to the penetration behavior of various types of fluids. Reactive fluids, such as acids, oxidizing agents, and others, react differently with different formations and produce conditions, such as heat of reaction, reaction product compositions, and other conditions indicative of the formation involved. By the present process, formations can be tested with respect to their reactivity toward various fluids. The components of drilling muds, and numerous other materials that can be manipulated in the form of solid or liquid components of pumpable fluids, tend to absorb or entrain the formationfluids that occur in earth formations. In the present process the formation fluids can be sampled by collecting samples of the dispersed mud cake and/ or fluids brought into contact with the formations. In addition, covering exposed formations by a cup that is pressed tightly against the borehole wall and filled with a light-weight rock-contacting fluid such as a gas allows the light fluid to penetrate into the formation. When such a light fluid has penetrated into a formation, stratification occurs and the light fluid rises above any heavier fluids in the formation. Thus the interior of the cup becomes a part of the space in which the stratification of the light fluid and the heavier formation-fluid takes place. As the formation fluid seeks its level, under the light fluid, it tends to fill at least the bottom portion of the cup with formation fluid. The formation fluid in the cup can then be collected and analyzed.

Numerous types of apparatus can be employed to subject a portion of a borehole wall to a sonic irradiation that disperses mud-cake in conjunction with a fluid displacement that contacts the exposed formation with a rockcontacting fluid of known composition. Any means of directing sonic radiation against a portion of the wall can be used in conjunction with any means of pumping in suflicient rock-contacting fluid to displace the borehole fluid. A particularly suitable apparatus comprises: a cup having a rim pressed against the formation and having ports in fluid communication with the cup interior; a sonic transducer mounted inside the cup and electrically connected to a source of high frequency current; and a rockcontacting fluid container provided with means for pumping fluid into the cup.

The above objects and advantages of this invention will be more easily understood from the following detailed description of preferred embodiments thereof when taken in conjunction with the attached drawing, in which:

FIGURE 1 is a vertical section of a borehole illustrating the use of an apparatus constructed in accordance with this invention;

FIGURE 2 is a vertical section of a borehole showing a second apparatus constructed in accordance with this invention which is provided with a means for measuring the permeability of the formation; and

FIGURE 3 is a partial vertical section of a borehole showing another embodiment of this invention which includes a means for measuring the permeability of the formation.

Referring now to FIGURE 1, there is shown a vertical section of a borehole is having a mud cake 1-1 disposed thereon. The mud cake 111 obscures the formation 9 traversed by the bore hole, making it substantially impossible to determine the characteristics of the formation 9. Disposed within the borehole is a logging instrument constructed in accordance with this invention which utilizes an outer casing or housing 12. The outer casing 12 is preferably liquid-tight and is secured to a suitable cable 13 in order that it may be lowered into the borehole and retrieved. The cable 13 in addition provides the various circuits required to couple the instruments in the housing 12 to the surface located instruments. The cable 13 in addition to providing required electrical circuits must have suflicient mechanical strength to support the instrument within the borehole.

Mounted on the right-hand side of the housing 12 is a cup-shaped member 14 provided with a resilient or pliable outer edge 15. The cupeshaped member 14- is mounted on a flexible bow spring member 17 while similar spring members 16 are positioned around the circumference of the housing 12 to center the instrument within the borehole and press the pliable outer edge of the cup-shaped member 14 firmly against the mud cake. When the cupshaped member is pressed against the mud cake it will effectively isolate an area 20 of the borehole. The isolated area may then be irradiated to remove the mud cake and the formation tested to determine its permeability or other characteristic.

Disposed at the center of the cup-shaped member 14 is a sonic generator or crystal 21. The crystal 21 should be disposed with its irradiating surface facing the borehole wall in order to irradiate the mud cake with sonic energy. The crystal 21 is connected to a high frequency generator 22 disposed within the housing 12 by suitable conductors. The high frequency generator 22 receives power from the surface over conductors disposed within the logging cable 13. The sonic generator 21 may be of any well known design, suitable sonic generators and their operation being described in the above-referenced copending application.

Disposed in the lower portion of the housing 12 is a fluid container 23. The container 23 is filled with a fluid 24 which is used for displacing both the formation fluid trapped within the interior of the cup-shaped member 14 and the formation fluid. The fluid 24 is preferably a fluid that is immiscible in both the formation fluid as well as the borehole fluid and in addition has known characteristics. In addition, the fluid should have a lower density than the borehole fluid and the formation fluid in order that the difference in density may be used to supply the pumping power to cause the fluid 24 to flow into the formation and thus displace the formation fluid. The pumping power for circulating the fluid 24 is obtained by utilizing the difference in densities between the fluid 24 and the formation fluid to cause a gravity flow of the two fluids. The formation fluid is removed from the cup member 14 by conduit 26 whose lower end 31 is disposed adjacent the bottom of the container 23 and whose upper end 36' is disposed on the lower side of the interior of the cup-shaped member 14. A second or equalizing conduit 25 is disposed with its lower end 27 adjacent the bottom of the container 23 and its upper end 28 passing through the side wall of the instrument housing 12. The contacting fluid 24 flows into the cup member 14 through a third conduit 3-2 whose lower end 33 is disposed at the top of the container 23 and'whose other end 34 opens into the upper portion of the cup-shaped member 14.

From the above description it is readily seen that the difference in densities between the =fluid 24- and the formation'fluid will cause the fluid 24 to pass upwardly through the conduit 32 to disperse the formation fluid which can then flow down the conduit 26 to the bottom of the container 23. The difference in density between the fluid 24 and formation fluid should be large enough to insure a substantial flow of fluid 24 into the formation. This fluid flow will insure that the mud cake removed from the wall of the borehole is flushed away or transported to the bottom of the container 23, thus directly exposing the portion of the formation isolated by the rim of the cupshaped member 14.

The difference in pressure between the borehole fluid and the formation fluid may be used to pump the fluid 24 into the formation to displace the formation fluid. This pressure difference may be used as the sole pumping power to circulate the fluid 24 or to add to the pumping power derived from the difference in densities between the fluids. Also while the fluid 24 is preferably immiscible in both the borehole and formation fluids a miscible fluid could be utilized by providing a flexible partition in the container 23 to separate the fluids.

In order to control the flow of the fluid 24 from the container 23 and limit its flow to desired positions within the borehole, solenoid operated valves 4% and 4d are disposed in the conduits 26 and 32, respectively. The solenoids of the valves 44 and 41 are connected to operating circuits at the surface (not shown in FIGURE 1) by means of circuits 42 and 43, within the cable 13.

When the logging instrument shown in FIGURE 1 is operated it is first lowered into the borehole by means of the cable 13. The instrument is lowered to the depth of the formation which is to be investigated. After the instrument is disposed at the selected depth the sonic generator 21 is energized to disperse the mud cake. At the same time that the sonic generator is energized the valves 4'9 and 41 may be opened to permit the fluid 24 to flow into the interior of the cup-shaped member 14 to displace the formation and borehole fluids trapped therein and thus transport the dispersed mud cake to the bottom of the container 23. The fluid 24 in addition to dispersing the formation fluid trapped within the cupshaped member 14- will flow into the formation which is exposed by the removal of the mud cake. This flow will result from the pressure differential between the borehole fluid and the formation fluid as well as the difference in densities between the fluids. As is well known in the art of well drilling, the borehole fluid is maintained under suflicient pres sure to insure that any flow will be from the borehole fluid intothe formation and thus prevent the possibility of the well blowing out during the drilling thereof. By timing the period during which the valves 4a and 41 are maintained open and measuring the amount of fluid 24 displaced from the container 23 one can determine the permeability of the formation. Of course, it will be necessary to correlate the above measurements with measurements made on formations having known permeabilities in order to accurately determine the precise permeability of the formation investigated. Such corre lating procedures are, of course, well known in the log ging art and are utilized frequently for correlating logged information with similar information obtained from known formations in order to relate the characteristics of the logged formation to known formations.

Referring now to FIGURE 2, there is shown a second embodiment of this invention which is capable of measuring the permeability of the formation. In this embodiment of the invention a cup-shaped member 54 is disposed on a bow spring 56 which is fastened to the housing 50 of the logging instrument. The housing 50 is provided with end caps 51 and 52 which thread into the housing to provide a liquid tight housing. Additional bow springs 57 are disposed around the circumference of the housing 5% in order to both center the logging instrument in the borehole and press the cup 54 firmly against the borehole wall. The sonic generator 55 is disposed at the center of the cup-shaped member and is coupled to a high frequency generator 60. The generator 60 is coupled to a power supply 58 at the surface by means of a circuit 5% disposed within the logging cable 53. The cable 53 of course contains the required electrical circuits as well as the mechanical strength for supporting the logging instrument within the borehole. The above described portion of FIGURE 2 is similar in construction and operation to that described with reference to FIGURE 1.

A fluid container 61 is formed in the lower portion of the housing 50 and is partially filled with a fluid 62 having similar characteristics to that described above with reference to the fluid 24. The lower portion of container 61 is filled with a mixture of the formation and borehole fluids 63. The interface 64 between the two fluids 62 and 63 will remain distinct as long as the fluids are immiscible in each other. In cases where it is desired to use a fluid 62 which is miscible in the formation fluid as explained above, a flexible diaphragm can be provided in the container 61 to maintain the fluids separated. Conduit 65 is provided for circulating the formation fluid from the interior of the cupshaped member to the bottom of the container in a manner similar to that described with relation to FTG- URE 1. The conduit 65 is provided with an opening 66 disposed in the lower portion of the interior of the cup-shaped member 54. A conduit 7i} having an end 71 opening into the interior of the cup member 54 is similarly provided for circulating the fluid 62 from the container 61 to the cup-shaped member 54. An equalizing port 72 is provided in the bottom end cap 52 of the housing to equalize the pressure between the fluid in container 61 and the fluid in the borehole. A valve 73 is disposed in the conduit 65 while valves 74 and 75 are disposed in the conduit 7% The valves 73 and 75 are valves capable of completely closing off conduits 65 and 7s, respectively, while the valve 74 only partially closes the conduit 74' In addition, the valve 74 should permit a calibrated flow through the conduit 70 and operate in unison with the valve 73. The solenoids of the individual valves 73, 74 and 75 are coupled to power supplies located at the surface by means of circuits 76, 77 and 78. The power supplies at the surface are provided with switches 81?, 81 and 82 for operating the valves 73, 74 and '75. While simple switches are shown for 80, 81 and 82, other control means may be used as well as a control cycle in order to open and close the valves in a predetermined timed sequence to obtain the desired information from the formation.

A pressure measuring device is disposed to measure the pressure differential existing between the fluid flowing in the conduits 65 and 7:), respectively. This pressure measuring device consists of an E-shaped magnetic core 83 disposed within the housing 549 having windings 84 and 85 disposed thereon. A flexible diaphragm as is disposed adjacent the ends of the pole pieces and is subjected on one side to the pressure existing in the conduit 65 and on the other side to the pressure existing in the conduit 70. Thus, the flexible diaphragm will move either toward or away from the ends of the pole pieces depending upon the pressures existing in the two conduits. The two coils 84 and 85 are coupled to an oscillator 87 which is coupled to a recording means 1 at the surface by means of a circuit 9i disposed within the cable 53. The oscillator 87 should be designed so that its frequency varies with changes in the inductance of the coils 84 and 85. The inductance of the coils 84- and 85 of course varies with the position of the flexible diaphragm -36. Thus, the frequency of the oscillator 87 will be directly related to the difference in pressure of the fluid in the conduits 65 and 70.

The apparatus described above with relation to FIG- URE 2 is designed to measure the permeability of the formation based upon the penetration of the fluid 62 into the formation after the mud cake has been removed. The formation permeability is measured by first removing the mud cake and circulating the fluid 62 in a manner similar to that described above with relation to FIGURE 1. After the mud cake is removed, the borehole fluid is displaced by opening the valves 73, 74 and 75. The fluid 62 is circulated until the formation fluid has been displaced and the valves 73, 74 and 75 are then closed. This shuts off the flow of fluid 62 from the container 61 but the fluid 62 will continue to penetrate or infiltrate into the formation due to the pressure differential existing between the fluid in the con- 6 duit 70 and the formation pressure. This in effect will lower the pressure in the conduit 70 to the formation pressure wlt'le the fluid pressure in the conduit 65 below the valve 73 will return to the borehole pressure. Accordingly, the diaphragm 86 will be moved towards the pole pieces of the core '83, thus changing the impedance of coils 84 and and the frequency of the Oscillator 87. This frequency can be measured on the recording means 91 at the surface and is related to the difference between the formation fluid pressure and the borehole fluid pressure and is referred to herein as Ap. After this measurement is obtained, the valve 75' is opened while maintaining the valves 73 and 74; closed. The fluid 62 will again flow into the formation due to the pressure differential existing between the borehole and the formation. Since the valve 74 is only a partially closed valve and has been precalibrated the pressure of the fluid 62 will be reduced as it passes through the valve 74. The pressure will be reduced in direct proportion to the velocity of the flow in the conduit 79,

which is of course related to the permeability of the formation. This velocity will be measured by the differential pressure-measuring device since the pressure in the conduit 7% will be less than the pressure existing in the conduit 65 and again the diaphragm 86 will be displayed to the left. The resulting change in the frequence of the oscillator 87 will be measured and recorded by the device 911 and will be related to the velocity v. Having determined both the velocity v and the difference in pressure Ap the formation permeability K can be calculated from the following formula wherein C is a constant to be determined by calibrat ing the instrument. This calibration can be accomplished by making the above measurements while the instrument is disposed adjacent a formation having a known permeability. Once the instrument is calibrated any formation may be logged and its permeability determined by the above-described means.

Referring to FIGURE 3, there is shown a modification of the instrument described above with relation to FIGURE 2 wherein electrodes are used to measure the resistance of the irradiated portion of the formation to determine its permeability instead of using a pressure differential measuring means to determine permeability. The cup-shaped member 14th is disposed on a bow spring member 1&1 and is urged into contact with the wall of the borehole by means similar to that described above and illustrated in FIGURE 2. The sonic generator 102 is disposed at the center of the cup-shaped member and irradiates an isolated section 105 of the formation. Conduits 1&3 and 194- are provided for conducting the formation contacting fluid to the cup-shaped member and removing the formation fluid therefrom in a manner similar to that described above. Two electrodes and 111 are disposed in the rim of the cup-shaped member 1% and are coupled to surface instruments by means of a circuit 112 disposed within the well logging cable (not shown in FIGURE 3). The circuit 112 is coupled to a generator 113 which supplies a potential to two electrodes 110 and 111. An ampmeter 114 is disposed in series with the generator 113 while a voltmeter 115 is disposed in parallel with the generator 113. Thus, one can accurately measure the resistance between the two electrodes 11? and 111. By using a formation contacting fluid having a known resistance one can determine the permeability of the formation 105 which is irradiated by the sonic generator 102. More particularly, the contacting fluid should have a higher resistance than the formation fluid and the resulting increase in the resistance between the electrodes 110 and 111 as the fluid flows into the formation to displace the formation fluid will be directly related to the permeability 7 of the formation. The exact correlation between the measured resistivity and the permeability of the formation must be determined by calibrating the instrument using a formation having a known permeability.

While but three embodiments of this invention have been described, it is obviously susceptible to many modifications and changes without departing from its broad spirit and scope.

I claim as my invention:

1. An apparatus for logging a formation surrounding a fluid filled borehole, wherein the formation is covered with a mud cake, said apparatus comprising: an instrument housing adapted to be lowered into a borehole; said instrument housing including an isolation means for isolating a portion of the formation from the fluid filling the borehole; a sonic generator mounted on said instrument housing and disposed to irradiate the portion of the formation isolated by the isolating means to disperse the mud cake therefrom; a container disposed in said instrument housing and filled with a rock contacting fluid, said rock contacting fluid having a lower density than the borehole and formation fluids; conduit means for transferring the rock contacting fluid from the container to the irradiated portion of the formation to displace both the borehole and formation fluids therefrom; valve means disposed in said conduit means to control the flow of rock contacting fluid and a logging instrument disposed on said instrument housing to measure a characteristic of the irradiated portion of the formation related to the displacement of the borehole fluid therefrom.

2. An apparatus for logging a formation surrounding a fluid filled borehole wherein the formation is covered with a mud cake, said apparatus comprisin an instrument housing adapted to be lowered into a borehole; a cup-shaped member mounted on said housing and having a resilient outer edge; biasing means disposed on said housing to urge the outer edge of said cup shaped member into engagement with the surface of the borehole to isolate a section thereof; a sonic generator disposed within said cup shaped member and directed to irradiate the isolated section of the borehole to disperse the mud cake therefrom; a container disposed in said housing and filled with a formation contacting fluid; a first conduit means connecting the upper portion of said container with the upper portion of said cup shaped member; a second conduit means connecting the lower portion of said container with the lower portion of said cup shaped member; valve means disposed in said first and second conduit means to control the flow therethrough, said valve means being controlled from the surface and means for measuring the flow through said conduits and transmitting a related signal to the surface.

3. A Well logging apparatus comprising in combination: a housing adapted to be moved through a borehole; a cup shaped member carried by the housing and having a rim capable of being disposed in substantially fluidtight contact with the borehole wall, said cup shaped member having at least one port in fluid communication with the interior thereof; a sonic generator mounted inside the cup shaped member and having a vibratory surface facing the borehole wall; a fluid container carried by the housing and provided with a conduit in fluid communication with the port on said cup member; said fluid container being filled with a formation contacting fluid having a lower density than the borehole and formation fluids, valve means disposed in said conduit to control the flow of formation contacting fluid into the cup interior; pressure-measuring means for measuring pressure-differentials created by the flow of fluid from the cup interior into the formation; and an indicating circuit connected to indicate the extent of the pressuredifferentials.

4. The well logging apparatus of claim 3 in which the fluid-container is carried by the housing at a position below the level of the cup shaped member and the valve means is a surface operatable valve means for controlling the flow of fluid through said conduit.

5. A well logging apparatus comprising in combination: a housing adapted to be moved through a borehole; a cup shaped member carried by the housing and hava rim capable of being disposed in substantially fluidtight contact with the borehole wall, said cup shaped member having at least one port in fluid communication with the interior thereof; a sonic generator mounted inside the cup shaped member and having a vibratory surface facing the borehole wall; a fluid container carrier by the housing and provided with a conduit in fluid communication with the port on said cup member; said fluid container being fill-ed with a formation contacting fluid having a lower density than the borehole and formation fluids, valve means disposed in said conduit to control the flow of formation contacting fluid into the cup interior; a pair of electrodes disposed at spaced locations in the rim of the cup member and circuit means for supplying a potential to said electrodes and measuring the change in the formation resistance between said electrodes as said fluid flows from the interior of the cup to displace the formation fluid.

6. A well logging apparatus comprising in combination: a housing adapted to be moved through a borehole; a cup shaped member carried by the housing and hava rim capable of being disposed in substantially fluidtight contact with the borehole Wall, said cup shaped member having at least one port in fluid communication with the interior thereof; a sonic generator mounted inside tne cup shaped member and having a vibratory surface facing the borehole wall; a fluid container carried by the housing and provided with a conduit in fluid communication with the port on said cup member; said fluid container being filled with a formation contacting fluid having a lower densi y than the borehole and formation fluids, valve means disposed in said conduit to control the flow of formation contacting fluid into the cup interior; and means disposed to measure both the velocity of flow of said fluid from the interior of said cup into the formation and the pressure difference between the formation fluid and the fluid filling the borehole.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN APPARATUS FOR LOGGING A FORMATION SURROUNDING A FLUID FILLED BOREHOLE, WHEREIN THE FORMATION IS COVERED WITH A MUD CAKE, SAID APPARATUS COMPRISING: AN INSTRUMENT HOUSING ADAPTED TO BE LOWERED INTO A BOREHOLE; SAID INSTRUMENT HOUSING INCLUDING AN ISOLATION MEANS FOR ISOLATING A PORTION OF THE FORMATION FROM THE FLUID FILLING THE BOREHOLE; A SONIC GENERATOR MOUNTED ON SAID INSTRUMENT HOUSING AND DISPOSED TO IRRADIATE THE PORTION OF THE FORMATION ISOLATED BY THE ISOLATING MEANS TO DISPERSE THE MUD CAKE THEREFROM; A CONTAINER DISPOSED IN SAID INSTRUMENT HOUSING AND FILLED WITH A ROCK CONTACTING FLUID, SAID ROCK CONTACTING FLUID HAVING A LOWER DENSITY THAN THE BOREHOLE AND FORMATION FLUIDS; CONDUIT MEANS FOR TRANSFERRING THE ROCK CONTACTING FLUID FROM THE CONTAINER TO THE IRRADIATED PORTION OF THE FORMATION TO DISPLACE BOTH THE BOREHOLE AND FORMATION FLUIDS THEREFROM; VALVE MEANS DISPOSED IN SAID CONDUIT MEANS TO CONTROL THE FLOW OF ROCK CONTACTING FLUID AND A LOGGING INSTRUMENT DISPOSED ON SAID INSTRUMENT HOUSING TO MEASURE A CHARACTERISTIC OF THE IRRADIATED PORTION OF THE FORMATION RELATED TO THE DISPLACEMENT OF THE BOREHOLE FLUID THEREFROM. 